FLORIDA STATE HORTICULTURAL SOCIETY, 1959
276
(8) Stevens, J. W., and Baier, W. E., Refractometric determination of soluble solids in citrus juices. Ind. & Eng.
Chem. Anal. Ed. 11: 447-449; 1939.
(9) Westbrook, G. F. and Stenstrom, E. C. A study of
the degree Brix and Brix/acid ratios of grapefruit utilized
by Florida Citrus processors for the seasons 1952-1953
through 1955-1956. Fla. State Hort. Soc. Proc. 70: 247-259;
1957.
MEASUREMENT OF COLOR CHANGES IN GREEN BEANS
A. L. Shewfelt1 and R. A. Dennison
Department of Food Technology and Nutrition
University of Florida
Gainesville
Variations in the color of green beans may
arise as a result of differences in variety, ma
turity, storage, and processing.
Since the
chlorophyll in green beans is relatively sensi
tive to heat, even mild blanching may cause
significant color change by converting some
of the chlorophyll to pheophytin.
Conversion of chlorophyll to pheophytin is
complete in most canned green beans. As a
result the canned product has an olive or
olive-brown color. Color of the product may
vary substantially in both shade and uniformity
from a muddy gray-brown to a clear bright
olive-green.
The olive-brown color is often present in
frozen green beans to a lesser extent than
in the canned product. Color of the frozen
product may vary substantially also.
This study was conducted to gather more
information about the basic color differences
that occur in green beans and to develop
simple and reliable methods of evaluating
these differences.
Literature Review
Smith and Benitez (3) reviewed methods
used to estimate chlorophyll and its derivatives
in plant materials.
Mackinney and Weast (2) measured the
chlorophyll breakdown in blanched green
beans by determining the optical density of
extracts at 535 and 560 mu. Dietrich et al
(1) estimated chlorophyll retention in frozen
beans by blending a sample with acetone, then
filtering and washing the residue with acetone.
Optical density readings of the filtrate were
made at 534 and 556 mu. Chlorophyll was
completely converted to pheophytin by adding
an oxalic acid solution to the filtrate and
Florida
Agricultural
No. 988.
xPresent
address:
Morden, Manitoba
Experiment
Canada
Station
Department
Journal
of
Series,
Agriculture,
allowing to stand overnight. Sweeney and
Martin (4) studied the effect of heat treat
ment on chlorophyll degradation in broccoli
and suggested that changes in absorbance at
665 mu could be used as an index of chlor
ophyll a retention.
The ratio of Gardner a to b color difference
readings for beans was used as an index of
color (1) and Gardner a readings and subjec
tive color ratings were found to be correlated
with chlorophyll retention (4).
Materials and Methods
Absorption spectra of acetone extracts
Duplicate 50-gram samples of deseeded
fresh, frozen, and canned green beans were
blended with 250 ml. of undiluted acetone for
3 minutes and filtered through Whatman No.
1 filter paper. Optical density readings of the
clear filtrate were recorded for the wavelength
range of 400 to 700 mu. Absorption curves
were compared with those of pure chlorophyll
a, chlorophyll b, pheophytin a, and pheophytin
b (3).
Measurement of chlorophyll a retention
The measurement of chlorophyll a retention
was based on the degree of reduction in
absorbance at a wavelength of 665 mu in
relation to the absorbance at a wavelength of
556 mu. The latter wavelength represented
the crossover point of the chlorophyll a and
pheophytin a curves.
A 50 gram sample of fresh deseeded beans
was blended with 250 ml. of undiluted acetone
for 3 minutes and filtered. The optical density
of the filtrate was measured with a Beckman
DU spectrophotometer at wavelengths of 556
and 665 mu. The ratio of optical density
readings at 665 and 556 mu was used as an
index of 100% chlorophyll a retention. A 100
ml. aliquot of the extract was treated with
1 ml. of 1% oxalic acid in 90% acetone and
allowed to stand overnight. The ratio of the
optical density readings at 665 and 556 mu
was used as an index of 0% chlorophyll a re
tention. A straight line relationship between
100% retention and 0% retention was used to
SHEWFELT AND DENNISON: BEAN COLOR CHANGES
estimate the chlorophyll retention in an un
known sample.
Measurement of color differences
For color difference measurements, a 50
gram sample of deseeded product was blended
277
with 100 ml. of distilled water for 2 minutes.
Color difference values were obtained on the
slurry by means of a Gardner Color and Color
1.60
FRESH BEANS
FROZEN BEANS
CANNED BEANS
1.60
PHEOPHYTIN
fl
FRESH BEANS•>
1.40
-FROZEN BEANS
CANNED BEANS
1.20
; 1.00 ■
il 1
ft
j\
z
UJI
CHLOROPHYLL a
H
I
!\ \i
J 0.60
1
f
M
' 0.40
V
0.20
0.00
1I
w
1 0.60
400
490
500
550
600
650
0.00
400
450
500
550
600
650
700
WAVELENGTH (Mp)
V,
700
WAVELENGTH (Mp)
2.00
1.80
1.80
1.60
1.60
FRESH BEANS
FRESH BEANS
FROZEN BEANS
CANNED BEANS
CHLOROPHYLL fc
1.40
FROZEN BEANS
CANNED BEANS
PHEOPHYTIN b
>- 1.40
z
UJ
LOO
-J 1.00
I
o
£0.80
o
0.60
0.60
0.4G
0.40 -
0.20
0.20
0.00
400
450
900
950
500
WAVELENGTH (Mm)
550
700
0.00
400
450
500
950
600
WAVELENGTH (Mp)
690
700
FLORIDA STATE HORTICULTURAL SOCIETY, 1959
278
Difference Meter standardized with a medium
green plate (Rd 25.2, a-28.4, b 7.7). The
slurry in the sample cell was mixed thoroughly
with a porcelain spatula immediately before
taking the readings in order to avoid error as
a result of settling. The readings were taken
within one-half hour of blending.
Comparison of different brands of frozen, green
beans
Samples of ordinary cut and French cut
frozen beans of three different brands were
defrosted, examined visually for color, and
subjected to the procedures described above
for chlorophyll a retention and color difference
measurements.
Comparison of varieties of green beans....
The varieties Blue Lake, Pearlgreen, and
Tendergreen were grown under comparable
conditions. Gardner readings were taken on
the fresh products of each and also on the
canned products processed for 20 and 60
minutes at 240 °F.
Results and Discussion
Absorption spectra of acetone extracts
Absorption curves for acetone extracts of
fresh, frozen, and canned samples are com
pared with the absorption curve for chlorophyll
a, chlorophyll 6, pheophytin a, and pheophytin
b and are illustrated in Figures 1, 2, 3, and 4
respectively.
The curve for the fresh bean extract had
significant peaks at 430, 580, 615, and 665
mu, and hence resembled closely the chlor
ophyll a curve which had peaks at the same
wavelengths (Fig. 1).
Table 1.
The curve for the frozen bean extract (Fig.
1) resembled that of fresh beans except for
the more prominent bulge at 410 mu and
the lesser absorbance at 665 mu. This indi
cated a partial conversion of the chlorophyll a
to pheophytin a.
The prominent peaks for the curve of the
canned bean extract were at 410, 532, 610,
and 665 mu (Fig. 1). This curve had the
same maxima as the pheophytin a curve (Fig.
3), with the exception that a peak in the
pheophytin a curve at 515 mu was not evident
in the canned bean extract.
A lack of similarity between the bean ex
tract curves and those of chlorophyll b and
pheophytin b (Figs. 2 and 4) gave rise to the
question of whether there was any significant
amounts of these compounds present. The
slight bulge at 455 mu in the curves of the
fresh and frozen bean extracts corresponded
to a prominent peak for chlorophyll b at that
wavelength. There was, however, no evidence
of a peak or bulge at 642.5 mu. In like man
ner, in the curve of the canned bean extract,
there was a very slight bulge at 435 mu which
may have been related to the very high peak
for pheophytin b (Fig, 4). but there was no
indication of an absorption maximum at 655
mu.
The above observations support those of
Sweeney and Martin (4) and indicate that the
chlorophyll b fraction of green beans is of
minor significance in routine examination of
color differences.
Chlorophyll * retention and color difference in commercial retail
samples of frozen green beans.
Chlorophyll'j.
Brand
A
Style
cut
French cut
Visual color
retention %
Gardner Reading
Rd
b
dull gray-green
47.3
50.7
12.2
-16.2
-16.3
19.6
dull med.-green
11*8
a
19.4
B
cut
French cut
aed. light, brown-green
med. dark, brown-green
23.6
25.0
12.4
11.3
-12.5
-15.8
18.3
IB.8
C
cut
French cut
med. bright-green
bright light-green
63.5
74.8
10.9
12.8
-15.2
-20.4
18.7
20.7
SHEWFELT AND DENNISON: BEAN COLOR CHANGES
Measurement of chlorophyll a retention
279
The usefulness of measuring chlorophyll a
retention is illustrated by the values obtained
Using a Beckman DU spectrophotometer,
for different brands of frozen green beans
(Table 1). The French cut beans were con
sistently higher in chlorophyll a retention than
the ordinary cut beans. The major differences
occurred, however, among the different brands.
Brand C was substantially highest and brand
B lowest in chlorophyll a retention. Although
the differences in visual color and chlorophyll
the optical density ratio 665/556 for 100%
chlorophyll a retention was found to be 11.80
and that for 0% retention was 7.25. These
values were used to construct the reference
curve in Figure 5. Since the values vary from
one instrument to another, a separate curve
would have to be established for each instru
ment or laboratory. For example, the com
parable readings using a Spectronic 20 instru
ment were 6.8 for 100% chlorophyll a reten
tion and 4.6 for 0% retention.
a retention were reflected to some extent in
the Gardner readings, the differences in the
latter were much less apparent.
Comparison of varieties of green beans. .
o
Although chlorophyll a retention values were
Pioo
useful in assessing the effects of heat and
LJ
storage treatments of fresh and frozen beans,
80
the Gardner values were useful in assessing
differences among varieties. Here, the fresh
products were assumed to have 100% chlor
ophyll a retention and the canned products 0%
60
o.
retention.
o
Gardner readings taken on the fresh product
(Table 2) showed that Tendergreen had a
lower reflectance and was thus darker.. Ten
dergreen was also lowest in Gardner a and
Gardner b thus indicating less intensity of
5*0
7
8
9
10
OPTICAL
ri«, i.
II
DENSITY
12
13
14
665/556
green and yellow pigments. The Rd values
were substantially higher for the processed
than for the fresh products. With processing
increased beyond 20 minutes the Rd values
for both Blue Lake and Tendergreen remained
relatively stable, although that of Pearlgreen
decreased and showed a darkening tendency.
Chlorophyll { rataMioa la term* of optical (tonally ratio Mt/U«
The measurement of chlorophyll a retention
was useful in following the rate of chlorophyll
breakdown as a result of heat treatment or
extended storage of fresh and frozen products.
The conversion of chlorophyll a to pheophytin
a was noted to be substantially complete after
15 minutes of processing at 240 °F and 95%
complete after 60 minutes at 205 °F.
Comparison of different brands of frozen green
beans
Table 2.
The Gardner a value of Pearlgreen persisted
at a higher level than that of Blue Lake and
Tendergreen as the processing time was in
creased. This indicated that the Pearlgreen
Gardner readings on the fresh and processed products of three
varieties.
Processed 20 min.
at 2jL0°F*
Fresh
Processed do min.
at 2fc°P.
Variety
Hd
a
b
Hd
a
b
Kd
a
b
Blue Lake
8.1
-16.2
18.3
12.0
-4-4
20 .8
12 .2
-3.2
20.8
Pearlgreen
8.2
-15.9
18.2
U.I
-5.0
18 .2
11 .4
-4.0
17.5
Tendergreen
6.2
-14.5
16.2
,11.4
-4.4
16 .6
11 .1
-2.4
17.5
FLORIDA STATE HORTICULTURAL SOCIETY, 1959
280
variety maintained a more intense green color
during processing.
In the processed products, the Gardner h
value of Blue Lake persisted as the highest
and that of Tendergreen was lowest. The cor
responding readings for Pearlgreen were gen
erally intermediate. In varietal comparisons,
it would seem preferable to consider the
Gardner a and h values separately rather than
rely on an a/b index.
A subjective examination of the processed
products showed that the Pearlgreen samples
were substantially lighter and clearer in color
than the other two varieties. The Blue Lake
processed samples were generally darker than
the other two varieties. The Tendergreen
samples had an opaque muddy-colored liquor.
Summary and Conclusions
Absorption spectrum curves for acetone ex
tracts of fresh, frozen, and canned green beans
showed that the majority of green pigment
variations were the result of differences in
chlorophyll a and its conversion products
rather than chlorophyll b.
A routine procedure for the estimation of
chlorophyll a retention was outlined. The
procedure was applicable for use with either
a Beckman DU spectrophotometer or a Spectronic 20 instrument.
Marked variations occurred in the chlor
ophyll retention of different samples of frozen
green beans. The chlorophyll retention test
was useful in evaluating the freshness and
storage history of fresh and frozen green beans.
Gardner color difference readings were use
ful in determining variations among varieties
and processing treatments. The variety Pearl
green had a numerically higher Gardner a
reading than comparable samples of Blue Lake
and Tendergreen. The Pearlgreen beans had
a clearer and brighter color when canned.
LITERATURE
CITED
1. Dietrich, W. C, Nutting, M. D., Olson, R. L, Lindquist, F. E., Boggs, M. M., Bohart, G. S., Neumann, H. J.
and Morris, H. J.
Time-temperature tolerance of frozen
foods. XVI. Quality retention of frozen green snap beans
in retail packages. Food Technol. 13:136-145. 1959.
2. Mackinney, G., and Weast, C. Color changes in green
vegetables. Ind. Eng. Chem. 32:392-395. 1940.
3. Smith, J. and Benitez, A. Chlorophylls: Analysis in
plant materials. In Moderne Methoden der Pflanzeranalyse.
Ed. Paech and) Tracey. Vol. 4 pp. 142-196. 1955. SpringerVerlag. Berlin.
4. Sweeney, James P., and Martin, Margaret. Determina
tion of chlorophyll and pheophytin in broccoli heated by
various procedures. Food Research 23:635-647. 1958.
PREVENTION OF THE BUTT DISCOLORATION OF
PREPACKAGED CELERY
times brownish.
C. B. Hall
Florida Agricultural Experiment Station
Representative samples can
be seen in Figure 1.
Gainesville
The discoloration or darkening of the butts
of prepackaged celery is considered a major
problem by celery prepackagers. It is felt
that the darkening detracts from the appear
ance and that potential consumers would
consider the celery not fresh.
The darkening occurs on the cut surface of
the butt with the largest amount in the vascu
lar ring area. In some cases only the vascular
ring area is affected, while in others the whole
surface is affected. Even when the whole
surface is affected, the vascular ring area is
usually much darker. Generally the discolor
ation is black or a shade of black, but is some
Florida
No. 964.
Agricultural
Experiment
Station
Journal
Series,
Figure 1. Control of butt discoloration of celery seven
days after treatment by (1) 1% sodium dehydroacetate
(2) 1.5% potassium sorbate (3) water control.